The present invention relates to a process for mechanically dewatering extracted sugar beet pulp.
The energy needed to remove a defined amount of water is several times lower in mechanical dewatering process compared to thermal drying in conventional drum driers. Increasing the proportion of press water and thus the dry matter content of the pressed pulp represents considerable energy savings. In addition, there is the fact that saving primary energy is a critical factor in decreasing emissions.
The single- and twin-screw presses of vertical and horizontal type currently in general use for pulp pressing operate in accordance with the volume displacement principle, wherein the applied pressure and the pressing time are the relevant operating parameters for pressing. Improving the dry matter content in the pressed pulp by decreasing the spindle speed (in order thereby to increase the pressing time) leads to a decrease in throughput. Increasing the dry matter content by prolonging the duration of pressing thus inevitably leads to higher capital costs.
In practice, particular importance is attached to the use of pressing aids. These are taken to mean those additives or measures which--without decreasing the throughput of the pulp press--increase the dry matter content achieved in the pressed pulp. Pressing aids are, in particular, acids and gypsum which are added to the fresh extraction water or press water.
The press water pH during pressing promotes pressing-off if it is in the region of around 5.0. In addition to improving pulp pressing, however, the disadvantages associated with such measures must be taken into account, e.g., the increased molasses production due to ion exchange processes and the formation of calcium lactate. With excessive acidification, the dry pulp yield and the filter station in the juice purification stage can be adversely affected.
Addition via the press water or fresh water of salts of polyvalent cations as pressing aids is currently considered a standard technique. A gypsum solution is predominantly used, in such a quantity that the unesterified galacturonic acids of the pectin are occupied by calcium ions. DE-A 29 06 528 discloses a two-step pressing process in which salt solutions of polyvalent cations are added in an intermediate stage between the two pressing stages. This process, however, decreases the process throughput and efficiency.
The current mechanical pulp dewatering techniques used in practice remove only approximately 80% of the water introduced with the extracted pulp, achieving a dry matter content in the pressed pulp of on average approximately 30%. The remaining 20% of water must be removed by drying.